Mutualism

Mutualism definition and examples in nature

By definition, symbiosis requires a close association between organisms of different species (close enough for them to help or harm each other), and in mutualism, both organisms have a net positive gain due to their relationship and this close association.

There are many examples of mutualism that can be observed in nature. A classic example that you may remember from your days of learning plant biology, is the mutualistic relationship between many plants and nitrogen-fixing bacteria.

To jog your memory, Nitrogen is present in very large amounts in the air (around 78% of the air is nitrogen, with oxygen making up 21% and the rest being mostly carbon dioxide and inert gases) but most plants cannot harness it themselves. Yet, nitrogen is still required in most plants to make chlorophyll (the green pigment required for photosynthesis) and many plant products, especially legumes (beans).

Because they cannot trap nitrogen directly from the air, legume plants turn to nitrogen-fixing bacteria like Rhizobium, which are located on the plants' roots and can grab nitrogen from the soil. Rhizobium does nitrogen fixation and shares this nitrogen with the plant, who then utilizes it. With all this newly accessible nitrogen, the plant can go into overdrive, creating its own products.

The legume plant makes many more beans, and it also makes more chlorophyll and can undergo more efficient photosynthesis. The efficient photosynthesis produces more photosynthates, which are important products of photosynthesis that can be used by both the plant and the nitrogen-fixing bacteria. The most important of the photosynthates are carbohydrates and sugars, which Rhizobium bacteria happily munch on.

Thus, the increased nitrogen availability due to the presence of bacteria leads to more chlorophyll and a hardier plant, which leads to more photosynthesis which leads to more carbohydrate plant products, which leads to happy, munching bacteria. We can see how this relationship is mutually beneficial for both species (Fig. 1).

Nitrogen fixing bacteria and mutualism.

Another example of mutualism will be very familiar to purveyors of children's films made in the early 2000s. The sea anemone and the clownfish display a mutually beneficial relationship because clownfish can live amongst the anemone's tentacles. (Yes, tentacles. Even though sea anemones look like plants, they are actually animals!).

The ocean floor, where clownfish live, can be quite a dangerous place, and sea anemones make the perfect hiding place for longer-term dwelling, or to dart in and escape from a predator. Not only can sea anemones somewhat conceal the clownfish, but more importantly, they are quite poisonous. The tentacles of sea anemones contain a potent neurotoxin that is dangerous to other sea-dwelling animals like plankton, fish, jellyfish, barracudas, and even sharks. Some of these animals are the predators of clownfish, so a quick dart into the embrace of the stinging tentacles of a sea anemone may save the life of a clownfish many times over.

Conveniently, clownfish themselves are impervious to the stings of the sea anemone neurotoxin, so they are not harmed by brushing against its tentacles. Clownfish and sea anemones are so closely related, that another name for clownfish is the anemonefish (Fig. 2)!

clownfish and anemones mutualism

We know the benefit that a relationship with the sea anemone has for the clownfish, but what is the benefit of this association for the sea anemone? Clownfish residing in the sea anemone cause movement of water, which is thought to lead to increased oxygenation for the sea anemone. Also, clownfish due to their coloring, attract some prey that the sea anemone can then use its toxic tentacles to kill before eating. As gross as it may sound, clownfish poop is very helpful to the sea anemone, acting as a nutrient-rich material to help algae grow within the anemone.

Algae and sea anemones have a mutually beneficial relationship of their own, where the algae provide sugars and carbohydrates to the anemone, and the anemone provides other nutrients and increased sunlight exposure to the algae.

Types of mutualism

Mutualistic relationships can be further parsed into specific types. The following are the broad categories of mutualism.

1. Resource-resource mutualism - this is a mutually beneficial relationship in which both parties provide the other with resources that each otherwise would have difficulty (sometimes impossibility) accessing. Also known as biologicalbarter

   • Example: Legume plants and nitrogen-fixing bacteria.

2. Service-resource mutualism - this is a mutually beneficial relationship in which one party does a service that benefits the other, while the other organism provides the first with resources in exchange.

   • Example: Plants that provide certain ants with nectar (the resource) in exchange for the ants living on the plants and protecting them from ladybugs that would otherwise devour the plant (the service).

3. Service-service mutualism - this is a mutually beneficial relationship in which both parties provide the other with services. This is quite rare.

   • Example: The sea anemone and the clownfish.

Mutualism vs commensalism, with human examples

We know how the definitions of mutualism and commensalism differ. In mutualism, both organisms benefit; while in commensalism only one benefits while the other remains neutral.

Yet while these definitions are quite succinct, the real-life distinctions are not so easily delineated. Especially in humans, some of the most complex of the organisms we have on this planet; we cannot always pinpoint when a form of symbiosis is either mutualism or commensalism.